1. Field of the Invention
The present invention is related to an optical amplifier apparatus capable of transmitting signal light in a bidirectional manner, to an optical transmission apparatus equipped with a break point detecting function using this optical amplifier apparatus, and also to a bidirectional optical transmission apparatus.
2. Description of the Related Art
In optical amplifiers using optical fibers as amplifying media, optical amplifying operations of these optical amplifiers are adversely influenced by light returned from reflection points existed inside these optical amplifiers and optical transmission paths. When such return light appears, the optical amplifiers own a problem. That is, the optical amplifying operations of these optical amplifiers are brought into unstable conditions, and in the worst case, the optical amplifiers would be oscillated. In general, in order to suppress such reflection/return light originated from this reflection point, optical isolators having coreactive effects are arranged at input/output units of an optical fiber amplifier.
However, when such an optical isolator is arranged, the optical fiber amplifier will have directivity due to the coreactive effect itself. Therefore, this optical fiber amplifier may cut off signal light traveling along a reverse direction opposite to the original travel direction. That is, as to signal light traveling along the original travel direction, the optical amplifying operation of this optical amplifier becomes effective, so that this signal light may be transmitted. However, as to signal light traveling along the reverse direction opposite to the original travel direction, this signal light is cut off in the optical amplifier. As a result, this optical amplifier cannot perform the bidirectional optical transmission. To avoid this drawback, a plurality of optical circulators are necessarily required to be combined with the optical amplifiers. Thus, the overall optical system would become complex. Furthermore, with respect to a unidirectional optical transmission, for example, there is another problem that conditions of optical fiber transmission paths cannot be checked by employing an optical time domain reflectometer (OTDR). In the case that a break point is detected so as to measure a transmission path loss occurred in an optical fiber transmission path and/or to perform maintenance of an optical fiber transmission path, an optical pulse is entered from a transmission end into this optical fiber transmission path, and a light level of a returned optical pulse is monitored in a time sequential manner. As a result, the transmission path loss of this optical fiber transmission path is detected, a check is made as to whether or not such a break point is present, and if such a break point is detected, then a distance up to this break point is measured.
However, as previously described, in the case that the optical amplifier containing the optical isolator is arranged in the optical fiber transmission path, since the return light is cut off by this optical isolator, it is not possible to detect another break point existed in the optical fiber transmission path subsequent to the optical amplifier along the light travel direction.
Also, in an optical communication system containing an optical amplifier, an upstream signal is cut off by means of an optical isolator built in the optical amplifying unit. Accordingly, a bidirectional optical transmission system cannot be established.
An object of the present invention is to provide an optical amplifying apparatus applicable to a bidirectional optical transmission, and also operable in such a case that while signal light is transmitted along an original transmission direction, signal light is wanted to be transmitted along a reverse direction opposite to this original transmission direction. For instance, a condition of as optical transmission path is checked by an OTDR.
An optical amplifier apparatus, according to an aspect of the present invention, is featured by comprising an optical amplifier for optically amplifying first signal light entered from an input port thereof and for outputting the first amplified signal light from an output port thereof, and a 4-port optical circulator. In this case, the optical circulator has a first port, a second port, a third port, and a fourth port. Then, light entered from the respective ports is sequentially outputted to the next port. The input port of the optical amplifier is connected to the second port, and the output port thereof is connected to the third port.
The optical amplifier apparatus of the present invention is further comprised of a first optical isolator arranged at a pre-stage of the optical amplifier, and a second optical isolator arranged at a post-stage of the optical amplifier.
Also, an optical amplifier apparatus, according to another aspect of the present invention, is featured by comprising an optical amplifier for optically amplifying first signal light inputted from an input port thereof and for outputting the first amplified signal light from an output port thereof, and an optical circulator equipped with at least four ports through which light is inputted/outputted, in which light inputted from a first port thereof is outputted form another port thereof. Then, the input port of the optical amplifier is connected to the second port among the four ports, and the output port of the optical amplifier is connected to the third port, the first signal light is entered to the first port for outputting light to the second port, second signal light is entered to the fourth port for outputting the light inputted to the third port, and the second signal light is outputted form the first port.
An optical transmission apparatus equipped with a break point detecting function, according to another aspect of the present invention, is featured by comprising the above-explained optical amplifier apparatus, a first optical transmitter module for transmitting the first signal light, and an optical time domain reflectometer for outputting an optical pulse along the same direction as that of the first signal light. In this case, this optical time domain reflectometer is comprised of an optical pulse light source for outputting the optical pulse, an optical coupler for coupling the first signal light to the optical pulse, and a light receiver module for receiving light returned by the optical pulse.
Also, a bidirectional optical transmission apparatus, according to another aspect of the present invention, is featured by comprising the above-described optical amplifier apparatus, a first optical transmitter module for transmitting the first signal light, a second optical transmitter module for transmitting the second signal light, a first optical receiver module for receiving the amplified signal light outputted from the optical amplifier apparatus, and a second optical receiver module for receiving the second signal light. In this arrangement, the bidirectional transmission apparatus is further comprised of a bidirectional optical amplifying relay device for optically amplifying the first signal light and the second signal light.
The optical amplifier apparatus of the present invention employs such an arrangement that the input and output sides of the optical amplifier are connected to a single 4-port optical circulator. With employment of such an arrangement, the signal light which should travel along the original transmission direction is detoured in such a manner that this signal light is entered via the 4-port optical circulator into the optical amplifier. On the other hand, the signal light which travels along the reverse direction opposite to the original transmission direction is shortcircuited in the 4-port optical circulator, while being not detoured along the direction of the optical amplifier, and then, is directly outputted to the optical fiber transmission path.
As a result, for instance, in the case that the break point of the optical fiber transmission path is intended to detect by using the optical time domain reflectometer (OTDR), the optical pulse may pass through the optical amplifier in the outgoing path along which this optical pulse is transmitted. On the other hand, the return light is shortcircuited, and then is returned to the optical time domain reflectometer without passing through the optical amplifier. Thereafter, the return light is received by the light receiving device. As a consequence, it is possible to carry out a test similar to the normal break point detection for the optical fiber transmission path.
Also, in the bidirectional optical transmission system, for example, the downstream signal light is caused to pass through the optical amplifier, and then this downstream signal light can be optically amplified. Even in this case, since the upstream signal light does not pass through the optical amplifier, this upstream signal light can be optically transmitted without being cut off. In this arrangement, the upstream signal light cannot be optically amplified. However, if the transmission capacity of the downstream line is made asymmetrical to that of the upstream line, and the optical amplifying operation is required only for the downstream line, the above-described arrangement may have effectiveness. In addition, by combining the optical amplifying relay device, which is capable of optically amplifying the signal light in a bidirectional manner, with the optical amplifier apparatus of the present invention, it is possible to optically amplify only the signal light that is needed with respect to each of the upstream/downstream lines.